Clarify that batching methods can be used with any orderable type primary key,...

Clarify that batching methods can be used with any orderable type primary key, not just integer ones, as per @a58cafeb

activerecord/lib/active_record/relation/batches.rb

@@ -40,8 +40,8 @@ module Batches
     #
     # NOTE: It's not possible to set the order. That is automatically set to
     # ascending on the primary key ("id ASC") to make the batch ordering
-    # work. This also means that this method only works with integer-based
-    # primary keys.
+    # work. This also means that this method only works when the primary key is
+    # orderable (e.g. an integer or string).
     #
     # NOTE: You can't set the limit either, that's used to control
     # the batch sizes.
@@ -90,8 +90,8 @@ def find_each(options = {})
     #
     # NOTE: It's not possible to set the order. That is automatically set to
     # ascending on the primary key ("id ASC") to make the batch ordering
-    # work. This also means that this method only works with integer-based
-    # primary keys.
+    # work. This also means that this method only works when the primary key is
+    # orderable (e.g. an integer or string).
     #
     # NOTE: You can't set the limit either, that's used to control
     # the batch sizes.